Number one suspicion so far is that a combination of an unbalanced load (due to lane closures for construction), heat (it's been unseasonably hot here lately with lots of days in the 90's), good breeze (10 gusting to 20, and that had to be amplified in the river valley), all added up to the collapse.

Those things piled onto a bridge that was built on the cheap. Documents already uncovered by some investigative journalists (so take it with a grain of salt until the actual investigators get their hands on them), indicate the bridge was considered not in top shape several years ago. The structural members were rated only 4 out of 9. An engineer interviewed said that is a rating that should cause serious concern. On top of that it is a truss bridge, not built with big huge steel beams as most bridges that size.

The stuff could really hit the fan when all the details of those inspection reports become widely available. The engineer said that kind of relatively light construction does not carry an unbalanced load very well; and also that no section can hold up well on it's own. Would explain why the whole thing came down very quickly after the center section failed.

Lots of speculation so far, but first indicators are that this was a poor design for the busiest bridge in the state. Certainly not strong enough to have ignored the problems found in those earlier inspections.

EDITED, removed an extra "not."

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Don of Borg - Cool, Calm, Collective.

"Within the next generation I believe that the world's leaders will discover that infant conditioning and narco-hypnosis are more efficient, as instruments of government, than clubs and prisons, and that the lust for power can be just as completely satisfied by suggesting people into loving their servitude as by flogging them and kicking them into obedience." -- Aldous Huxley